How to get post's current parent term ID?

How do you get the ID of the parent term of the current child term assigned to a post. For a custom taxonomy?

Custom taxonomy: “location”

Countries

  • Japan
  • USA
  • Mexico

Cities

  • New York
  • Austin

Say we are on a post assigned to “Japan” how do I get the ID of the parent called “Countries”? For this case it doesn’t matter if it takes the highest parent or just the direct parent.

I found this:

$term_id = 21;

$child_term = get_term( $term_id, 'location' );
$parent_term = get_term( $child_term->parent, 'location' );

But it doesn’t work.

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